In the conventional cases of removing the scattered gamma-ray noise in a gamma camera, a main measurement energy window and two subsidiary measurement energy windows are set in an energy spectrum obtained by measurement, the scattered gamma-ray noise in the main measurement energy window is estimated on the basis of scattered gamma-ray noise measured in the two subsidiary measurement energy windows, and the estimated scattered gamma-ray noise is removed from the amount of the gamma-rays measured in the main measurement energy window. (See, for example, Patent Literature 1.)
The above manner of estimation and removal of the scattered gamma-ray noise is often called the TEW (triple energy window) technique. In this specification, the TEW technique is explained in detail in connection with and in comparison with the embodiment.